1. Field of the Invention
This invention relates to an improved method of bleaching cellulosic pulp in a first chlorination stage of a multistage bleaching process. More particularly, the invention relates to a method of bleaching cellulosic pulp in the first chlorination stage of a multistage bleaching process, using chlorine and chlorine dioxide, wherein the chlorine dioxide charge is split in a manner such that the total amount of environmentally unfavorable by-products can be reduced, without any significant impairment of the cost effectiveness and bleaching results.
2. Description of the Prior Art
The bleaching of cellulosic materials, particularly woodpulp, has in the past been the subject of extensive study and experimentation. As a result, numerous processes for bleaching cellulosic materials are known and used in commercial paper making operations. The more common processes employ more than one reagent and more than one bleaching step in the bleaching operation. Some of these multistep processes involve, for example, treating the cellulosic pulp with a reagent such as chlorine, washing the residual chlorine and solubilized impurities from the treated pulp with water, extracting the chlorinated lignins with caustic and washing with water, treating the cellulosic pulp with another bleaching agent, such as chlorine, chlorine dioxide or sodium hypochlorite and again washing the residual reagent and solubilized impurities from the treated cellulosic pulp. Many variations on the multistage bleaching process are well known.
The first stage of a multi-stage bleaching process for cellulosic pulp (sometimes hereinafter referred to as "the first chlorination stage") is intended to remove residual lignin and other impurities from the pulp. Various oxidizing agents, including chlorine and chlorine dioxide or mixtures thereof have heretofore been employed in this first chlorination stage for the purpose of removing the lignins and other impurities remaining in the woodpulp after digestion. For example, it has previously been suggested in U.S. Pat. No. 3,536,577 to effect the bleaching of cellulosic materials, including woodpulp, using an aqueous solution of chlorine dioxide and chlorine at moderate temperatures. It has also been found that the characteristics of the pulp bleached by the latter process can be improved by the sequential application of chlorine and chlorine dioxide, rather than applying a mixture of the two. These processes are described, for example, in U.S. Pat. Nos. 3,433,702 and 3,501,374. More recently, it had been found that the efficiency of delignification of the bleached pulp could be improved if part of the chlorine is included with the chlorine dioxide in the first step of a bleaching application, followed by application of aqueous chlorine without an intermediate washing. See U.S. Pat. No. 4,432,598.
The bleaching of cellulosic pulp with bleaching agents including chlorine increasingly uses chlorine dioxide as a replacement for some of the chlorine. This is, in major part, because chlorine dioxide is considered to be less harmful to the environment. Chlorinated organic compounds are generated during pulp bleaching with chlorine or chlorine containing compounds. In particular, certain potentially toxic compounds, including tetrachloro-pdibenzodioxin (TCDD) and tetrachlorodibenzofuran (TCDF), (sometimes also referred to as dioxins) as well as Absorbable Organic Halides (AOX) are formed by the chlorination of their precursors, which are typically found in pulp mills. When chlorine in the first stage of bleaching is replaced by an equivalent amount of chlorine dioxide, the production of chlorinated dioxins, along with other chlorinated compounds is decreased.
The current trend is to express organically bound chlorine as AOX rather than TOCL (total organically bound chlorine). The difference in the two methods of expressing organically bound chlorine is in the analytical procedure. TOCL has all of the volatile chlorinated organic compounds removed before analysis, whereas AOX includes both the volatile and non-volatile chlorinated compounds. Consequently, AOX is almost always larger than TOCL. The ratio of AOX to TOCL is quite variable and appears to depend upon the type of pulp, whether or not oxygen delignification has been used, the bleaching process used, and other factors. The ratio generally is 1.0 to 1.4.
AOX appears to be a linear function of the total elemental chloride consumed. Individual chlorinated organic compounds such as TCDD and TCDF exhibit more complicated (curvilinear) relationships. Thus it cannot be assumed that a change in AOX will result in a corresponding change in TCDD and TCDF. AOX essentially measures high molecular weight material and therefore is a "relatively blunt" instrument for monitoring and controlling the discharge of bioaccumulating and toxic persistent substances.
In the aforementioned processes, wherein an amount of chlorine is substituted with chlorine dioxide, chlorine dioxide is generally added ahead of the chlorine (D/C) in the first chlorination stage at substitution levels of 30% or higher. This sequence of chlorine addition had been found to give the most economic bleaching response. Recently, it has been reported, however, that the formation of TCDD and TCDF can be minimized if chlorine is added ahead of the chlorine dioxide (C/D), even though this sequence of addition is not particularly cost effective. The trend of increasing substitution has, on an overall basis, resulted in an increase in the total cost of the bleached pulp. Thus, while each of these methods has certain advantages and disadvantages, it has not been possible to optimally utilize the bleaching chemicals chlorine and chlorine dioxide, to reduce the amount of environmentally unfavorable by-products such as chlorinated dioxins, while at the same time minimizing the impairment in cost effectiveness and bleachability.
Accordingly, it is an object of the present invention to provide a method of bleaching cellulosic pulp in a first chlorination stage using chlorine and chlorine dioxide which maximizes the reduction of environmentally unfavorable by-products formed in a multi-stage bleaching process, without substantially reducing bleachability and cost effectiveness.
Another object of the present invention is to provide a method of bleaching cellulosic pulp in a first chlorination stage which involves splitting the charge of total chlorine into three separate charges.
A still further object of the present invention is to provide an improved method of bleaching cellulosic pulp in the first chlorination stage of a multi-stage bleaching process which includes splitting the charge of chlorine dioxide so that chlorine dioxide is added to the pulp both before and after the addition of the chlorine, thereby effecting a reduction in the amount of environmentally unfavorable by-products compared to knows sequences of chlorine and chlorine dioxide addition in a first chlorination stage.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appendec claims, and upon reference to the accompanying drawings.